Conventionally, there has been known an air pollution control system for treating flue gas discharged from a boiler installed in a thermal power generation plant or the like. The air pollution control system includes NOx removal equipment that removes nitrogen oxides from flue gas discharged from a boiler, an air heater that recovers heat of flue gas having passed through the NOx removal equipment, a precipitator that reduces dust in the flue gas after heat recovery, and a desulfurizer that reduces sulfur oxides in the flue gas after dust reduction. As the desulfurizer, a wet desulfurizer that reduces sulfur oxides in flue gas by bringing a limestone absorbent into gas-liquid contact with flue gas has been generally used.
Waste water discharged from a wet desulfurizer (hereinafter, “desulfurized waste water”) contains various types of harmful substances, for example, ions such as chlorine ion and ammonium ion and mercury in large amount. Therefore, these harmful substances need to be removed from the desulfurized waste water before the desulfurized waste water is discharged to outside of the system. However, a removing process of these various types of harmful substances contained in the desulfurized waste water is complicated, and treatment cost is high. Therefore, to reduce the treatment cost of the desulfurized waste water, there has been proposed a method of reusing the desulfurized waste water in the system without discharging it to the outside of the system. For example, Patent Literature 1 discloses an air pollution control system in which a device that atomizes and gasifies desulfurized waste water is separately installed, branched from a flue gas duct of a main line that connects NOx removal equipment, an air heater, a precipitator, and a desulfurizer, and after a part of flue gas is introduced from the flue gas duct of the main line into the device, and desulfurized waste water is atomized into flue gas in the device and evaporated to precipitate harmful substances, the flue gas is returned to the flue gas duct of the main line.